This invention generally relates to color compatible aqueous thickener polymer compositions, their method of manufacture and method of use.
Aqueous polymer systems, for example aqueous coatings compositions comprising emulsion polymer binders, typically use thickeners to obtain the desired degree of viscosity needed for the proper formulation and application of the aqueous system. One general type of thickener used in aqueous polymer systems is referred to in the art by the term “associative.” Associative thickeners are hydrophobically modified so as to provide an aqueous system with certain characteristics such as ease of formulation, pigment settling prevention, film build during application, spatter resistance, low sag, good flow, and leveling. These associative thickeners may have hydrophobic groups attached to or within the water-soluble polymeric backbone. The polymeric backbone may be constructed from one or more blocks of polymerized oxyalkylene units, typically polyethylene oxide or polypropylene oxide. They may also utilize a cellulosic backbone.
The rheological response of associative thickeners upon color tinting is important in evaluating the quality of the thickener. This is so because associative thickeners typically exhibit a large Stormer viscosity (expressed in Krebs Units “KU”) drop when colorant is added. This is especially problematic when the paint will be tinted to a deep tone because a high level of surfactant generally accompanies the colorant. There is a need in the art for thickeners that maintain desired viscosity levels in an aqueous system upon the addition of colorant.
The orientation of hydrophobic groups along the thickener backbone appears to influence the color compatibility of associative thickeners. United States Patent Application Publication No. 2005/0150418 discloses a hydrophobically modified polymer with blocks of closely clustered hydrophobic groups for use in tinted aqueous systems. Although thickeners with bunched hydrophobic groups show a particular advantage in preserving viscosity upon tinting, they have acute disadvantages. As the examples in United States Patent Application Publication No. 2005/0150418 demonstrate, aqueous solutions of known thickeners with bunched hydrophobic groups must be prepared at solids concentrations that are too low for practical commercial utility. Solutions made at higher solids concentrations are not pourable. Organic solvent can be added to provide an acceptably high solids solution at a pourable viscosity. However, organic solvent is a volatile organic compound and its use must be minimized to meet current regulatory requirements. Modified cyclodextrins have been used to suppress the as-supplied solution viscosities of associative thickeners. However, extraordinarily high levels of cyclodextrin are required with associative thickeners containing bunched hydrophobic groups, and the cost is prohibitive. Surfactants can be used to suppress as-is viscosity, but surfactant concentrations similar to the concentration of the thickener must be employed. Using such quantities of surfactants is expensive, counterproductive to thickening efficiency, and can degrade final dried coating properties.
Most of the associative thickeners presently on the market are sold as pourable aqueous liquids. For ease of use, it is desirable for the viscosity (Brookfield at 6 rpm) of such thickener products to be less than 15,000 centipoise (cps), or even less than 5,000 cps, so that the product will readily drain from its storage container, and be readily incorporated into the aqueous system to which it is added. Obtaining such pourable viscosity from thickeners incorporated at high active solids concentration requires methods that lower viscosity. A number of methods are available to perform this function, but all of these methods have significant drawbacks.
A need in the art remains, therefore, for pourable associative thickeners with both low viscosity and the highest active thickener solids possible. A need also exists for a hydrophobically-modified thickener that allows for a superior rheology upon the addition of colorant while utilizing a cost-effective, environmentally friendly method to suppress the aqueous product viscosity.